In an optical transmission system, as a digital hierarchy for multiplexing an existing service signal, a synchronous digital hierarchy (SDH) has been internationally standardized.
In the United States, a synchronous optical network (SONET) having the same frame structure as the SDH has become an US national standard. An optical system that conforms to the SDH/SONET specification is the mainstream of a current optical transmission system and has been introduced all over the world. In recent years, an optical transport network (OTN) (for example see Non-patent Documents 1 and 2) has been standardized as a platform for transparently transmitting various clients such as an asynchronous transfer mode (ATM), an Ethernet (a registered trademark, hereinafter the same) as well as the SDH/SONET based on a wavelength division multiplexing technique that can cope with an explosive increase in Internet traffic. The optical transport network is expected to become the mainstream of a future optical transmission system.
Further, due to the explosive spread of the Internet, an Ethernet interface has abruptly increased, and in 2007, shipments of the Ethernet interface exceeded shipments of the SDH/SONET interface. As an Ethernet signal of a Giga class, a 1 Gigabit Ether (1 GbE) signal of 1.25 Gbit/s and a 10 Gigabit Ether (10 GbE) LAN PHY signal of 10.3125 Gbit/s have been standardized. In the future, as a client signal of a communication carrier, the 10 GbE is expected to be the mainstream. Further, a demand for connecting LAN environments located at remote sites through a LAY-PHY is increased.
As a method of satisfying the demand, on an OTN of a 10 Gb/s class, an OTU2e has been documented in ITU-T G.Sup.43 as a technique of mapping a 10 GbE-LAN PHY with an over-clocked OTU2 and transferring it and is being used as a de facto standard. In this case, the bit rate of the 10 GbE-LAN PHY is 10.3125 Gb/s±100 ppm, and the bit rate of the OTU2e is 11.096 Gb/s±100 ppm.
A configuration of a conventional transmission system and a frame structure of the OTN will be described with reference to FIG. 11. When a client signal S1 is input to an optical transmitter 4, an optical reception unit 41 receives the client signal. An OTU frame generation unit 42 converts the received signal to an OTU frame, and the OTU frame is transmitted through an optical transmission unit 43. When the OTU frame signal is transmitted along an optical transmission line 6 and input to an optical receiver 5, an optical reception unit 51 receives the OTU frame signal. A client signal extraction unit 52 extracts a client signal from the received signal, and the client signal is transmitted through an optical transmission unit 53. This signal is a client signal S2.
In the frame of the OTN, a stuff process control byte (hereinafter, a justification control (JC) byte) and a byte for stuff insertion at the time of a positive stuff (hereinafter, a positive justification opportunity (PJO) byte; a positive stuff byte) are defined in an overhead of an optical channel payload unit (OPU) inside an optical channel transport unit (OTU) frame. Further, a byte for stuff storage at the time of a negative stuff (hereinafter, a negative justification opportunity (NJO) byte; a negative stuff byte) is defined.
In asynchronous mapping accommodation in which a clock of the client signal is not synchronized with a clock of the OTN signal, a positive or negative stuff process corresponding to a frequency difference between the client signal and a signal of a payload portion of the OTN is performed, and accommodation into the OTN frame is performed. A frame of the G.709 standard assumes that a degree of bit rate accuracy of the client signal and a degree of bit rate accuracy of the signal at the OTN side are ±20 ppm, respectively. In the case of the OTU2e, since a clock of the 10 GbE-LAN PHY that is the client is synchronized with a clock of the signal at the OTN side, accommodation into the OTN frame can be performed without performing the stuff process.
FIG. 12 illustrates a frame structure at the time of optical channel data tributary unit (ODTU) multiplexing specified in ITU-T G.709. Columns 1 to 14 are an overhead of an optical channel transport unit (OTU) and an optical channel data unit (ODU), columns 15 and 16 are an overhead of an optical channel payload unit (OPU), columns 17 to 3024 are a payload of the OPU, and columns 3825 to 4080 are an error correction area. The OPU payload area is divided into 16 tributary slots (TSs). For example, in the case of mapping 4 ODU2 frames with an OPU3 frame of 40 G, multiplexing is implemented by dividing 16 TSs into four and allocating them to each ODU2.
In the G.709 frame, 1 byte (NJO) is prepared as a negative stuff byte, and 2 bytes are prepared as positive stuff bytes (PJO1 and PJO2). Thus, 3 stuff control bytes for supporting a stuff control status are prepared, and the stuff control status is controlled by a majority vote from the three bytes. FIG. 13 illustrates a definition of the G.709. For the JC bytes, a first row to a third row of a column 16 are set, and only 2 bits, a bit 7 and a bit 8, are defined. When the bit 7 and the bit 8 are “00,” it means “no stuff”. In the case of “01,” it means “negative stuff”, and in the case of “10,” it means “double positive stuff”. In the case of “11”, it is assigned to “positive stuff”. In FIG. 12, the positions of PJ01 and PJ02 change by a multi-frame alignment signal (MFAS) and the TS.
In the case of the frame structure illustrated in FIG. 12, it is possible to cope with a relative clock deviation from about −95 ppm to about +101 ppm by one negative stuff bit and two positive stuff bits. In this case, if a degree of clock accuracy at a network side and a stuff effect accompanied with multiplexing are excluded, a degree of clock accuracy at a client side is in a range of between −75 ppm and 81 ppm. In this case, it is difficult to accommodate the ODU2e signal having a degree of accuracy of ±100 ppm.